An example of a known construction in this field is the U.S. Pat. No. 2,771,773. In a patent U.S. Pat. No. 2,832,218 the theoretical background is given and in patent EP 0 146 902 the deduction of the equation is included. The equation is the mass flow q=M/n(R.sub.2.sup.2 -R.sub.1.sup.2) where M=torque, n=axis speed of rotation, R.sub.2 =an outer radius of the impeller, and R.sub.1 =inner radius from which the particles or the flow meets the impeller surface at or around its centre.